Conventional lawn mowers having upper and lower cutting units are manufactured so that the blades of the upper and/or lower cutting units are in the shape of saw-teeth or comb-teeth which project away from the axis around which the units rotate. Because of their shape the blades do not uniformly and smoothly cut the lawn at right angles to the direction in which the mower travels. As a consequence, the mowing area is small compared to the size of the upper or lower cutting units. Furthermore, the lawn is mown unevenly and, therefore, the mowed area is irregular and not uniform. In addition, such a mower does not cut the lawn sharply. Finally, the lawn that is taken in between the two blades is apt to slip out of the space between the two blades and, therefore, the mowing efficiency is low.
In order to overcome the drawbacks mentioned above, these lawn mowers have been redesigned so that both blades slide in parallel, on top of each other. Such a mower, however, has additional disadvantages. For example, it is difficult to position the blades so that they slide close to each other. Furthermore, the blades are apt to bite each other and therefore they are often damaged. In addition, because the load applied to the mower fluctuates, depending on the amount of the law cut by the blades, a large prime mover or motor is necessary to power the mower.
The conventional mower of the type mentioned above, has a vertical shaft which supports the upper and lower cutting units. Thus, the cutting units are suspended from the shaft and therefore, they are apt to rock. When rocking of the blades occurs, the height of the lawn that is mowed becomes uneven. Furthermore, when two or more of such cutting units are arranged in the same plane, there is often a space on the lawn which is left unmowed between the units, or the cutting units may come into contact with each other and break. Furthermore, when a supporting arm is provided to prevent rocking of the cutter units, the screws for attaching the cutter units to the supporting arm are apt to become loose due to the sliding movement of the cutter units when they mow the lawn. As a consequence, the cutter units rotate and become defective. It is therefore an object of the present invention to provide a lawn mower having blades that cut the lawn sharply and do not rock, that smoothly cut grass at right angles to the mowing direction, and that cut the lawn uniformly without the blades becoming damaged.